I used to do a lot of support on the Dell users forums. Dell DOES NOT support Vista or Windows 7 on this model (only drivers for XP). If it works on Win 7, but no sound you will have to do what others with Dell's that are not supported have done, install a PCI sound card that is Windows 7 compatible.
The sound in Dell's in the era of the Dim 3000 will only work with a Dell supplied sound driver. A non-Dell will either not work or not work correctly.
There are many posts on the Dell forum about this subject and older Dell's that are not supported by Dell.